Fabrication of embossed diffractive optics with reusable release agent

ABSTRACT

A method for producing a plastic material containing a diffractive optic pattern therein including embossing a curable plastic material with a diffractive optic pattern by pressing the curable plastic material against a master having a diffractive optic pattern thereon which is coated with a thin layer of a fluorinated silane, having at least one fluorinated alkyl group bonded to an Si atom and at least one alkoxy or halide group bonded to an Si atom, as a release layer. Also, the master coated with the fluorinated silane and cured plastic, particularly epoxy resins, containing diffractive gratings prepared by the method, preferably on a substrate, such as a waveguide.

This is a divisional of application Ser. No. 08/687,048 filed Aug. 1,1996, now U.S. Pat. No. 5,861,113.

The invention is directed to a novel method for inexpensively andreproducibly preparing diffractive optics. By chemically modifying acommercial master or submaster diffractive optic, particularly agrating, with a very thin layer of a release agent it is possible to usethe master or submaster to emboss replica gratings onto a variety ofsubstrates having a curable plastic surface, particularly waveguides,and to reuse the master or submaster multiple times for such embossing.The invention is also directed to the master or submaster containing thethin layer of release agent and to the substrates containing thediffractive optics prepared by the process.

BACKGROUND OF THE INVENTION

Planar optical waveguides are an attractive tool for use in analyticalchemistry and spectroscopy. A wide variety of inorganic and organicmaterials have been used to fabricate thin-film waveguides, and as aresult, planar guides can be engineered for specific chemicalapplications. As the evanescent wave is easily accessed, a number ofpapers have addressed the use of planar waveguides for bio/chemicalsensors. Attenuation, fluorescence, and interferometric sensors havebeen reported, as has the use of waveguides for enhanced Ramanspectroscopy.

Unlike fiber optics, planar waveguides have been slow to be widelyaccepted due to the difficulty of coupling light into the waveguide. Inthe laboratory, prism coupling is the predominant method, followed byend fire and grating coupling. Prism coupling, which operates on theprinciple of frustrated total internal reflectance, and endfirecoupling, which uses fiber optics or a lens to introduce light directlyinto the polished endface of the waveguide, are highly efficientmethods, as typically 80% of the laser beam is coupled into thewaveguide. The use of prisms and fibers does not damage the waveguide,and the various elements (prisms, fibers, and lenses) are reusable. Theyare impractical for routine use, however, as both coupling methodsrequire expensive positioning equipment. Prism coupling is sensitive toenvironmental fluctuations and destroys the two-dimensional geometry ofthe planar waveguide. Diffraction or reflection gratings for lightcoupling into planar waveguides are more practical than prisms or fibersfor routine use. Although the coupling efficiency observed with gratingsis reduced, the two-dimensional nature of the guide is conserved andgratings are generally more robust than prisms. Furthermore, the coupledpower is immune to environmental fluctuations because the grating isoften embedded in the waveguide.

Grating couplers are commonly fabricated using techniques based onholography. This approach involves an exposure step using a singlemirror which creates an interference pattern between two spatial halvesof a laser beam. The exposed photoresist acts as a mask for chemicaletching of the underlying waveguide or substrate to form a periodicgrating structure. This process can be time consuming, since this methodinvolves an exposure followed by a chemical etch. Blazed gratingsrequire additional fabrication steps. The use of an embossing techniquewhere the surface relief pattern of a master grating is pressed in to asuitable material may provide a fast and economical method to formgrating couplers for routine use.

Several investigators have published methods to emboss gratings forwaveguide applications. The earliest was Wei et al. (Wei, J. S.; Tan, C.C. “Coupling to film waveguides with reusable plastic gratings”, Appl.Op., 1976, 15, 289.) who used a thick (>100 μm) film of a polycarbonatethat was poured onto a master grating. The polycarbonate film wassubsequently peeled from the master grating and “stuck” on the waveguidesurface. Although this method is easy, it is not amenable to massproduction. Furthermore, reduced efficiency is observed due to the useof an extremely thick polycarbonate film and poor contact between thegrating and the waveguide surface.

This was followed by the work of Lukosz (Lukosz, W.; Tiefenthaler, K.“Embossing technique for fabricating integrated optical components inhard inorganic waveguiding materials”, Opt. lett. 1983, 8, 537-539) whoembossed gratings into sol-gel glasses. Although this technique uses amaster grating to impress a replica into a thin film guide, it islimited to sol-gel glass type waveguides. Furthermore, subsequent work(Roncone, R. L.; Weller-Brophy, L. A.; Weisenbach, L.; Zelinski, B. J.J. “Embossing gratings in sol-gel waveguides: pre=emboss heat treatmenteffects”, J. Non.Cryst. Solids 1991, 128, 111-117) showed that thegrating pattern was not uniformly transferred and that blaze (gratingprofiles) was distorted.

Christensen and Dyer (Christensen, D.; Dyer, S.; Herron, J.; Hlady, V.“Comparison of robust coupling techniques for planar waveguideimmunosensors”, Proc. SPIE, 1992, 1796, pp. 20-25) improved theembossing technique by coating the master grating with a vacuumdeposited aluminum film. The grating pattern is replicated onto thewaveguide surface with a UV curable epoxy. Because the aluminum filmdoes not adhere strongly to the grating, it “releases” the master fromthe cured epoxy replicate. This type of grating replication techniquecan be applied to all waveguide types. The limitation however is thatthe master grating is not truly reusable, for each new grating embossedthe aluminum release film must be reapplied to the master grating. Massproduction, and the resulting economies of scale, are thus impossible.

SUMMARY OF THE INVENTION

An object of the invention is to provide a process for the production ofdiffractive grating structures onto a substrate, particularly opticalwaveguides. The gratings allow the in and out coupling of light into thewaveguide or other substrate structure. The known methods forfabricating gratings, such as holographic exposure techniques, aretime-consuming, expensive and not amenable to easy high volumeproduction. This invention provides a process whereby gratings can beprovided on substrates in an inexpensive and reusable manner such thatsingle use sensors containing such gratings are much more economicallyfeasible. The invention provides a master grating with a release surfacewhich can be used for the embossing multiple gratings, for example up toseveral hundred, without the need for reapplication of the releaselayer. Further, inexpensive curable plastic materials, particularlyepoxy resins, are used for forming the grating on the substrate.

Upon further study of the specification and appended claims, furtherobjects and advantages of this invention will become apparent to thoseskilled in the art.

For achieving the above objects, the invention provides a method forproducing a substrate with diffractive optics thereon comprisingembossing into a curable plastic material contained on the substrate adiffractive optic pattern by pressing against the curable plasticmaterial a master diffractive optic pattern which is coated with a thinlayer of a fluorinated silane as a release layer, curing or otherwisehardening the plastic material and removing it from the master. Also,the invention provides a master diffractive optic comprising the masterdiffractive optic pattern, preferably on a substrate, which pattern issurface coated and modified with a fluorinated silane release agent.Further, the invention provides the product substrate with an embosseddiffractive optic pattern contained in a cured plastic material,preferably a cured epoxy resin.

It is preferred that the master diffractive optic pattern be provided ona substrate. The combination of master diffractive optic pattern andsubstrate may be those commercially available or may be a submasterprepared from the master by the embossing process of the invention orsome other process. The substrate of a commercial master diffractivegrating is generally a glass substrate having a diffractive patternthereon, however, the nature of the substrate is not critical and may beany of various materials including ceramic, polymer or metal material.The diffractive pattern may further be provided with a thin metalcoating. The coating must be sufficiently thin so as to avoid filling inor significantly diluting, the periodicity or shape of the diffractiveoptic pattern. Preferably, the metal coating is <1000 Å thick,particularly from 20 to 100 Å thick. The preferred metal coating is analuminum coating, but, other metals which provide surface hydroxylgroups may be used such as silver, platinum or gold . The metal layer isapplied to the surface of the diffractive optic pattern preferably byvapor phase deposition. Similarly, a layer of semiconductor material,such as Si or inorganic glass such as SiO or SiO₂ may be used as alayer.

The diffractive optic pattern is not particularly limited. In thepreferred embodiment it is a diffractive grating useful for the couplingin or out of light to or from a waveguide. They may have various gratingprofiles of different line spacing and groove shape, for example,sinusoidal or triangular. Examples of such diffractive gratings include1200 lines/mm and 2400 lines/mm gratings. However, spacing of a singleline of submicron width up to 3600 lines/mm could be used. Thus, themethod may be used for embossing channels or single ridges which are notnecessarily considered diffractive optical patterns.

Preferably a submaster of the master diffractive optic pattern isprepared and the submaster used for the embossing process. The submasteritself can be prepared by the embossing process of the invention byapplying the fluorinated silane release layer to the master diffractiveoptic, applying the curable plastic to another substrate, pressing themaster diffractive optic pattern onto the plastic to emboss the patterntherein, curing the plastic and providing a metal layer over theembossed pattern on the cured plastic. Then the fluorinated silanerelease layer is provided on the metal layer resulting in a new master,i.e., a submaster, for the embossing process. In this way the originalmaster diffraction pattern can be preserved and used for preparing otheridentical submasters, if necessary. The substrate for the submaster ispreferably glass, but, again may also be other materials such asceramic, polymer or metal.

The release layer is provided on the master or submaster by coating witha fluorinated silane to provide a thin layer thereon. The layer must besufficiently thin so as not to significantly fill in the surfacevariations of the diffraction pattern. Accordingly, the fluorinatedsilane layer should be provided in a thickness ranging from a molecularmonolayer of about 5 Å to up to about 1000 Å, particularly from 20 Å to100 Å, depending on the nature of the diffraction grating pattern andthe presence or absence of a metal layer. Such a thin layer of thefluorinated silane is preferably provided by vapor depositiontechniques. Although, transfer of a compact monolayer to a metal orglass surface by Langmuir-Blodcett techniques could also be used.Methods for vapor deposition of fluorinated silanes are known in the artor can be provided analogous to known methods. For example, vapor phasesolutions of 0.1 to 10% (v/v) are provided, and the master or submasterelevated above the solution so that the vapors derivitize the surface toprovide the layer.

The silane groups of the fluorinated silanes react with oxide groups oneither the substrate surface having the diffractive optics patternthereon or the metal layer coated thereover to provide a relativelystrong covalent bond of the release layer to the master or submaster.For example, with a glass substrate, the silane forms an —Si—O—Si— bondwith the silicon oxides of the glass. Thus, when the master or submasteris not coated with a metal layer the substrate should be selected toprovide suitable oxide groups for bonding with the silane functions ofthe release layer. Similarly, when using a metal layer, the metal mustbe selected to provide suitable oxide groups. Aluminum is the preferredmetal layer because it readily provides surface hydroxyl groups. This isalso why, when the submaster is prepared by embossing in an epoxy resin,a metal layer is provided thereover having oxide groups. As a result ofbonding to the master or submaster diffractive pattern or metal layersurface thereon, the release layer is not removed during the embossingprocess and can be reused many times.

The fluorinated silanes preferably are those having at least threecarbon atoms up to polymeric fluorinated silanes, such as TEFLON®.However, the lower molecular weight fluorinated silanes are preferred,i.e. those with 3 to 20, particularly 3-12, carbon atoms. Thefluorinated silane contains at least one, preferably one to three,alkoxy or halide group(s) bonded directly to the Si atom of the silanecapable of reacting with oxide groups on the substrate or metal layer toform the bond thereto as discussed above. Further, the silanes containat least one fluorinated alkyl group bonded to the Si atom of thesilane. Preferably the fluorinated alkyl group(s) have at least half oftheir hydrogen atoms replaced with fluorine atoms up to beingperfluorinated. For instance, monomeric silanes useful herein includethose of the following formula:

Si(R¹)_(a)(OR²)_(b)(X)_(c)

where R¹ is fluorinated alkyl group, preferably of 3-20 carbon atoms, R²is an alkyl group, preferably of 1-4 carbon atoms, more particularly 1or 2 carbon atoms, X is a halogen group, a+b+c=4, a≧1 and b+c≧1.However, silanes with multiple Si atoms may also be used ranging up topolymeric fluorinated silanes such as TEFLON®. When the fluorinatedsilanes are monofunctional, i.e. having only one alkoxysilane or halidesilane group, a monolayer of such silanes will be formed on thesubstrate surface or metal layer surface. If the silane ismultifunctional, particularly trifunctional, the silanes will not onlybond to the substrate or metal layer but also crosslink amongstthemselves in which case a thicker and more strongly adhered layer willbe provided. Thus, trifunctional silanes, for example(tridecafluoro-1,1,2,2-tetrahydrooctyl)-1,1-trichlorosilane, may bepreferred for some applications.

The curable plastic used for pressing against the master or submasterhaving the fluorinated silane layer thereon to emboss the diffractionpattern therein may be any curable plastic suitable for this purpose.For example, which cures or hardens to a material which does not causesignificant attenuation of the incident source, such as beingtransparent. Preferably, the curable plastic is a curable epoxy resin.Also preferably, the plastic is a UV-curable resin, although resinscurable by heat or two-part curable resins may also be used. When aUV-curable resin is used it is preferable that the substrate of themaster or submaster and/or the substrate of the embossed product be ofUV transparent material so that the resin can be cured by UV lighttransmitted therethrough. Particularly preferred UV-curable epoxy resinsare commercially available or preparable by methods know or analogousthereto. For example, UV-curable epoxy No. 81 from Norland Products Inc.(New Brunswick, N.J.) is useful herein.

Like the substrate for the master or submaster, the substrate for theproduct is generally a glass substrate, however, it may also be selectedfrom any of various other materials including ceramic, polymer or metalmaterial. In a particularly preferred embodiment, the substrate of theproduct-is a waveguide, especially a planar waveguide, and the embosseddiffraction grating provided thereon by the process facilitates thecoupling in or out of light to or from the waveguide.

In an alternate embodiment, further described in the Examples belowrelating to a specific embodiment for silicon nitride waveguides, thesubstrate for the product is another master or submaster containing adiffractive optic pattern thereon and a fluorinated silane release layerso that the product is a cured plastic resin having an embosseddiffractive optic pattern on both sides.

Another alternate embodiment can be used to prepare polymer waveguides.Therein, a submaster prepared as described above with a fluorinatedsilane release layer thereon is coated with a thin metal layer in amanner analogous to that described above. Then, the metal layer coatedsubmaster is pressed onto a curable plastic on a substrate to emboss thediffractive optic pattern therein. After curing of the plastic, thesubstrate is separated with the metal layer released from the submasterand adhered to the cured plastic. A polymer waveguide is then spun overthe metallized diffraction grating in a manner known in the art toprovide a polymer waveguide.

The invention can be used to provide diffractive optics on a widevariety of substrates in an inexpensive and reproducible manner. Forexample, the invention can be used to provide gratings on waveguides,couplers, narrow-band filters, beam splitters, focusing elements andeven on bulk optics to provide a diffraction effect when desired. Thus,the invention can be used in place of more expensive andnon-reproducible holographic or lithographic means. The invention isalso applicable to any embossed structure requiring a template, i.e.channels, or steps.

Particularly, the invention is useful for application to planar opticalwaveguides which are used in analytical and chemical detection methods.For example, such devices can be used for bio/chemical sensors,attenuation, fluorescence and interferometric sensors, enhanced Ramanspectroscopy, pH measurement and blood chemistry analysis. The abilityto produce the inventive waveguides with the coupling diffractiongratings in an inexpensive and reproducible manner makes single-useversions of sensors for such applications more feasible. Further, theinvention is useful for the reproduction of useful diffractive optics orother patterns.

The entire disclosure of all applications, patents and publications,cited above and below, and also of the entire article of AnalyticalChemistry, Vol. 68, No. 7, pp. 1245-49, is hereby incorporated byreference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) depicts a step index or ion-diffused waveguide embossed with adiffractive grating according to the invention.

FIG. 1(B) depicts a polymer waveguide embossed with a diffractivegrating according to an alternate embodiment of the invention.

FIG. 1(C) depicts a silicon nitride or other high refractive indexwaveguide embossed with a diffractive grating according to a furtheralternate embodiment of the invention.

FIG. 2 depicts the diffracted orders from an embossed epoxy transmissiongrating onto the top of a waveguide for θ_(i)=0°.

In the foregoing and in the following examples, all temperatures are setforth uncorrected in degrees Celsius; and, unless otherwise indicated,all parts and percentages are by weight.

EXAMPLES

Examples

Reagents and Materials

UV curable epoxy, No. 81, was obtained from Norland Products Inc. (NewBrunswick, N.J.). No. 81 was chosen for its rapid curing rate, hightransparency between 400 and 3000 nm, and refractive index of 1.56.Reflection gratings (1200 line/mm, 17° 27′ blaze angle) and glass prisms(Schott glass SF-2, η_(d)=1.644) were purchased from Edmund Scientific(Barrington N.J.). Cubic zirconia prisms (Lot No. PRE 0302, η_(d)=2.158)were purchased from Precomp (Great Neck, N.Y.). Microscope slides wereobtained from Erie Scientific (Portsmouth, N.H.) and Kimble (Toledo,Ohio). Polystyrene (MW=280 000, lot No. 00320EF) was purchased fromAldrich (Milwaukee, Wis.).(Tri-decafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane was fromPetrarch Systems (Bristol, Pa.). Sodium nitrate, silver nitrate,ammonium hydroxide, and, hydrogen peroxide were all of reagent grade.All materials were used as received.

Instrumentation

The UV source (365 nm) used to cure the epoxy gratings was fromSpectroline, Model enf-260c, 115 V, 60 Hz, 0.20 A (Spectronics Corp.,Westbury, N.Y.). Waveguide characterization was performed on a MetriconModel 2010 Prism Coupler (Metricon Corp., Pennington, N.J.).Measurements obtained in the laboratory employed a 5 mW, linearlypolarized HeNe laser (Uniphase, Monteca, Calif.) and a photodiodedetector (PIN-10D type, Model 818-SL Newport Corp., Irvine, Calif.) witha transimpedance amplifier (Model 101C, United Detector Technology,Hawthorne, Calif.). Interference measurements for polystyrene waveguideswere obtained with a UV-vis 260 spectrophotometer (Shimadzu, Kyoto,Japan). The spin-coater was constructed in-house using a variable speedHST110 motor controller (G. K Heller Corp., Floral Park, N.Y.).

Preparation of Waveguides

Green float glass waveguides were obtained as microscope slides fromErie Scientific. Glasses fabricated by the float process commonly havetin oxides incorporated into the surface. The presence of the oxidelayer increases the refractive index at the surface of the glasssubstrate, resulting in a layer that supports one or more guided modes.Silver ion diffused waveguides were fabricated by immersing cleanmicroscope slides (Kimble) in a 0.25 wt % AgNO₃/NaNO₃ solution at 320°C. for 15 min. The guides were removed, cooled, and thoroughly rinsed indeionized H₂O. Any precipitated (reduced) silver was removed by wipingthe surface with HNO₃. Polystyrene waveguides were fabricated byspin-coating. A clean microscope slide was flooded with a 50 mg/gsolution of polystyrene in toluene that was previously filtered with a0.2 μm stainless steel frit. Excess polymer solution was spun off, andthe film was formed by spinning at 2000 rpm for 1 min. The resultingwaveguides were allowed to dry in the presence of toluene vapors for 1day before use. Silicon nitride waveguides were fabricated in-houseusing standard silicon foundry procedures. A 1.8 μm thick SiO₂ bufferlayer was grown on a 3 in. p-type silicon wafer with a (111)orientation. Dichlorosilane was reacted with a stoichiometric excess ofammonia in a low-pressure chemical vapor deposition (LPCVD) process at750° C. to form silicon nitride layers between 1200 and 3400 Å thick ontop of the oxidized buffer layer. These wafers were then annealed at1000 ° C. for 60 min in flowing nitrogen. Following the first annealingstep, a 1400 Å layer of low-temperature oxide (SiO₂) was deposited onthe Si₃N₄ waveguide. The final annealing step was performed at 1000° C.for 45 min in flowing nitrogen.

Fabrication of Embossed Gratings. Ion-Diffused and Step IndexWave-guides.

Commercial gratings were silanized from vapor phase with a 1% (v/v)solution of (tridecafluoro-1,1,2,2,-tetrahydrooctyl)-1,1-trichlorosilanein toluene at 50° C. for 35 min. The gratings were allowed to dry atroom temperature for several hours. Embossed gratings were fabricated byplacing a small drop of UV curable epoxy onto the waveguide surface andpressing the epoxy drop onto the silanized master grating. Referring toFIG. 1A, for thin (0.1-2 mm) glass substrate (3), it is possible to curethe epoxy through the substrate with a UV lamp. After a 1 min exposure,the glass substrate is pulled away from the release-coated master,leaving an embossed grating (1) on the waveguide surface (2). Thisprocedure was used to fabricate gratings on float glass and ion-diffusedand step index waveguides.

Submasters

To preserve the commercial (master) gratings, all waveguide gratingswere embossed from copies of the master grating known as “submaster”(SM) gratings. The submaster was formed by first embossing a large (12mm diameter) epoxy grating onto a glass substrate, which wassubsequently aluminized (˜100 nm) by vacuum deposition. The aluminizedgrating was then silanized with the trichlorosilane release reagentdescribed above. Embossed gratings were then fabricated as describedabove. Gratings prepared from the SM exhibited diffraction efficienciesidentical to those exhibited by gratings prepared directly from thecommercial master.

Polymer Waveguides

Referring to FIG. 1(B), gratings were prepared by evaporating a thin(˜100 nm) coat of aluminum (4) on a SM grating. The coated SM was thenpressed onto a small drop (<1 μL) of epoxy placed onto the substrate aspreviously described. After exposure, the SM was removed, leaving an˜0.6 cm² aluminized grating (5) embossed onto the glass substrate (3).The polymer waveguide (6) was then spun over this replica grating.

Silicon Nitride and High Refractive Index Waveguides

Referring to FIG. 1(C), grating fabrication for Si₃N₄ and highrefractive index waveguides employs two SM gratings which have beenaluminized and silanized. A drop of epoxy is sandwiched between the twoSM gratings and exposed to UV. The SM gratings are pulled apart, leavingthe embossed grating gently adhered to one of the SM grating surfaces.The grating produced by this method is double-sided (7) (grating profileon each side). It is transferred to the silicon nitride waveguide (8) bypressing the waveguide against the grating and lifting it from the SMsurface. Alternatively, 2400 line/mm gratings can be used in the normalembossing orientation to couple directly into these high-indexwaveguides. When 1200 line/mm grating are used for guides with arefractive index above 1.8, the coupling must be accomplished through acladding (see below). If, however, a 2400 line/mm grating is used,normal grating fabrication as described above can be performed.

Characterization of Embossed Gratings. Scanning Electron Micrographs

SEM of the top of an embossed gratings prepared above, show a regularperiodic structure is obtained that lacks any significant defects. Thegroove spacing and blaze angle closely match the values reported for themaster gratings (0.83 μm, 17° 27′). Micrographs obtained for crosssections of the grating reveal that only the surface of the epoxy dropis embossed with periodic groove structure; the —unmodulated portion ofthe epoxy grating is 17 μm thick, while the modulated portion groovedepth is equal to 0.2 μm.

Diffraction Efficiencies

A beam incident upon a grating is diffracted into one or more ordersfollowing the relation:

d(sinθ_(i)−sinθ_(m))=mλ,m=0,±1,2,3, . . .   (2)

where d is the grating period, θ_(i) is the angle of the incident light,θ_(m) is the angle of the diffracted light, and m is the order ofdiffraction, as illustrated in FIG. 2. The general quality of theembossed gratings could be assessed in situ (on the waveguide) bymeasuring the efficiency of diffraction into the first-ordertransmission mode with a normal incidence beam. The diffractionefficiency for the m=+1 transmitted order of 10 replicate 1200 lines/mm,500 nm blazed, embossed gratings was determined to be 4.59%(u_(i)=0.10%) at 633 nm with an incidence angle of 0°. This diffractionefficiency was obtained as the ratio of the intensity of the m=+1transmitted order to the initial beam intensity incident upon thegrating, corrected for the front surface fresnel reflection loss. Thetotal diffraction efficiency of all orders, +1 transmitted andreflected, was determined to be 13.01% (u_(i)=0.54%). By way ofcomparison, the diffraction efficiency of a 1200 lines/mm embossed copyof a holographic grating (sinusoidal profile) was about 4.8% for m=1 atan incidence angle of 0°.

Grating Coupler Throughput Efficiency

The conditions for phase-matched grating coupling into a waveguide arewell known:

β=κ₀N_(c)sinθ_(i) +m2π/d

where β=κ₀N_(eff)=κ₀N_(w)sinθ_(w), κ₀=2π/λd, N_(c) is the refractiveindex of the cladding, θ_(i) is the incidence angle of the beam throughthe cladding, m is the diffraction order, d is the grating period (1200line/mm, d=0.833,um), Nw is the waveguide refractive index, θ_(w) is theangle of beam propagation in the waveguide, and d is the grating period.Coupling efficiencies (for the m=+1 order) for float glass waveguidesusing grating in-and out-coupling were measured for both substrate andsuperstrate (air) coupling. The highest efficiency for grating couplingis usually obtained with the laser incident from the superstrate whenthe grating is on top of the waveguide, as in the case of float glassand Ag+waveguides. The throughput for superstrate coupling was 0.54%(μ_(i)=0.17%) for eight waveguides, while substrate coupling throughputwas 0.43% (μ_(i)=0.06%) for the same sample set. The throughputefficiencies for the +1 order include input and output coupling loss,waveguide loss for the 2 cm path length, and diffraction efficiency ofthe grating. The total optical throughput found in this study iscomparable to those reported in the literature for grating couplersformed by photolithographic techniques.

Similar results were obtained for the Ag⁺-diffused waveguides, withthroughput efficiencies of 0.26% (u_(i)=0.08%) for four samples measuredfor the +1 order into mode 0 with superstrate coupling. With polystyrenewaveguides, however, adhesion of the embossed epoxy grating and thepolystyrene film was not uniform, and coupling was extremelyinefficient. Gratings embossed onto the glass substrates and thenovercoated with the spun polymer waveguide also failed to show anysignificant coupling. In an attempt to enhance the coupling efficiency,a buried aluminized grating was fabricated between the spun polystyrenewaveguide and the glass substrate. As previously described (FIG. 1B),the gratings were coated with an aluminum reflection layer, over whichthe polymer guide was spun. After this modification, waveguides modeswere observed in the polystyrene waveguide with throughput efficienciesof 1.12% (u_(i)=0.14%) for the +1 order (mode 0, with superstratecoupling, n=5). Although the 1.12% throughput efficiency obtained withgrating couplers is lower than that seen with prisms, the throughputreproducibility is significantly enhanced.

Input coupling was not observed for 1200 lines/mm gratings embossed ontothe Si₃N₄ waveguides in the normal orientation, as the groove period islimited to coupling into effective indexes below 1.76. As mode 0 has aneffective index of 1.895, either a higher frequency grating is required,or the incident beam must be launched through a cladding. Launching thebeam through a cladding can be accomplished by “flipping” the gratingupside down onto the waveguide surface, as depicted in FIG. 1C. Afterthis modification, waveguide modes were observed in the silicon nitridewaveguide with a throughput efficiency of ˜0.2% for the +1 order.Although this procedure yields a double-sided grating, this did notappreciably affect the coupling angles, and recent work suggests thatmore efficient coupling may result with properly designed double-sidedgratings. Waveguiding was observed with directly embossed single-sided2400 lines/mm gratings; however, the coupling was so weak that nooutcoupled beam was readily measured.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. An optical element having a diffractive opticpattern embossed in a cured plastic material without a metal layer onthe pattern, prepared by embossing a diffractive optic pattern into acurable plastic material by pressing against the curable plasticmaterial a master having a diffractive optic pattern thereon, saidmaster having a release layer coating of a fluorinated silane in athickness ranging from a molecular monolayer to up to about 1000 Å, saidsilane having at least one fluorinated alkyl group bonded to an Si atomand at least one alkoxy or halide group bonded to an Si atom.
 2. Theelement of claim 1, wherein the fluorinated silane layer is applied byvapor phase deposition.
 3. The element of claim 1, wherein thepreparation further comprises curing the plastic and releasing the curedplastic containing the diffractive optic pattern embossed therein fromthe master.
 4. The element of claim 3, wherein the plastic is athermoplastic and is cured by UV light.
 5. The element of claim 1,wherein the curable plastic is an epoxy resin.
 6. The element of claim1, wherein the curable plastic is provided on a substrate and curedthereon.
 7. The element of claim 6, wherein the substrate is a glasssubstrate.
 8. The element of claim 6, wherein the substrate is awaveguide.
 9. The element of claim 6, wherein the substrate is a planarwaveguide.
 10. The element of claim 1, wherein the fluorinated silane isof the following formula: Si(R¹)_(a)(OR²)_(b)(X)_(c) where R¹ isfluorinated alkyl group, R² is an alkyl group, X is a halogen group,a+b+c=4, a≧1 and b+c≧1.
 11. The element of claim 1, wherein thefluorinated silane contains three halide and/or alkoxy groups bonded toan Si atom.
 12. The element of claim 1, wherein the layer of fluorinatedsilane has a thickness of 20 to 100 Å.
 13. The element of claim 1, whichfurther comprises an additional diffractive optic pattern embossed intoan opposing side of the curable plastic material by pressing against thecurable thermoplastic material another master having a diffractive opticpattern thereon which is coated with a thin layer of a fluorinatedsilane, having at least one fluorinated alkyl group bonded to an Si atomand at least one alkoxy or halide group bonded to an Si atom, as arelease layer to obtain a thermoplastic material containing twodiffractive optic patterns on opposing sides.
 14. The element of claim13, wherein the plastic material containing two diffractive opticpatterns on opposing sides is provided on a silicon nitride waveguide.15. The element of claim 1, wherein the diffractive optic pattern is adiffraction grating of 1 to 3600 lines/mm.
 16. The element of claim 1,wherein the fluorinated silane has only one alkoxy or halide groupbonded to an Si atom and forms a release layer coating having amolecular monolayer thickness.